Electrode



R. CORTESE June 14, 1949.

ELECTRODE 2 Sheets-Sheet 1 Original Filed `May 1'7, 1945 'Rdlrh Co riem INVENTOR. y Wwf/f H\ S ATTORNEY `lune 14, 1949. R. CORTESE 2,473,413

ELECTRODE original Filed May 17, 1945 2 sheets-sheet 2` l RALPH COR T555 N/ BY al HIS AT TOR/VE Y /N VEN Tol? y Patented June 14, 1949 UNITED STATES PATENT? GFFICE 2,473,413

ELECTRODE Ralph Cortese, Newark; NI'J.'

Original application-May 17,' 1945, Serial Noi Divided :and misi-application:August 5, 1947, Serial No.1 766,145 r 3 Claims.

1 This application is a division of my copending application, Serial No. 594,226, filed May 17, 1945, issuedMay 11, 1948, as U. S. Patent No. 2,441,260, and entitledElectrode, and .the invention .relates tov fluorescent tubev lighting, and similarspace. discharge equipment, and morerparticularlyconcerns an electrode construction for the cold cath-- ode operation of fluorescent tubes.

Anotherobject is to provide an electrode struc-- ture for space discharge tubes operating,V under cold cathode conditions, Whichstructure contributes appreciably and without detrimental effect thereon,` in the Vpreliminary processing: of the tubes by the internal bombardment methodv and which iscapable'of satisfactorily operatingthe substantially the entire `length'of'the associated-V tube contributes to the production of useful light, wherein sputtering in large measure is avoided, and `inl the use of Which electrode both striking and `operating voltages are minimized.

Otherxobjects and'advantages in part will be obvious and in part pointed "out hereinafter dur`^ ing. the-course of the following description, taken in the light of the accompanying drawings.

My `invention accordingly resides in2the'several' combinations of elements,4 features of construction,- and association of parts,` as wellasin the relation of each of'th'e: same with one. or more of the others, the scope of the applicationof-'all of which is more particularly pointed out in the claims'at `the end of this specication';

In thedrawingswherein I disclose several embodiments of my invention,

Figure 1 comprises a fragmentary verticalview;

partly-in section'and partly in elevation, disclosing'the-embodiment of my invention which I pre` fer at present,

Figures 3 and 4 -fragmentarily disclose in ver: tical elevation, transverse section of line` 3--3` of` Figure 2,. and .longitudinal section the details Vor asecond-embodiment of my invention; whilev Figure 5 discloses in schematic illustration certain phenomena .attendant upon the practiceof my invention.

Figure 6 is aside-- elevation of a complete-tube employing paired' electrodes constructed according to the-embodiment disclosed in Figure 1.

Like reference characters denote like parts inl the several views.

'To-permit more ready and thorough-under'- standing of my` invention,l t.maybe notedat this -point that incomparatively recent 'years; somewhat revolutionary changes have occured in the illumination. art. In large measure, the incandescible...l'amentary type of lamp is being rapidly replacedsforawide variety of uses by the uorescent lamp.'Y

Despite the high. degree of vreiinement to which the ncandescible lamp .had heretofore. been .developed,.its very nature, whereby it relies for its entire. emission..upon the production of intense.

heat,-Av denitely limited/the theoretical maximum efficiencies .Whic'lfi could vbe achieved thereby.

Theintroduction ofthe fluorescent lamppermitted radical broadening voi the. possibilities available .in .illumination engineering. The almostcomplete-.energy conversion into the visible light .spectrum..with the substantial VeliminationA of.inra-redradiation.. in large measure removes thepresence vvof vsensible heat.. Theoretical eicienciesare increased.. Noteworthy nood-lighting,-now.is available.:` Batteriesof 2, 3 or 4 or more'. lamps. cam-be arranged. .for desired light-V ing eiects. Moreover, novel. arrangementsY are available whenntwoor more batteries. of. tubes` are employed.in.combination.A Effective color combinations-.can- `be.achieveddirectly by selectively employing lamps' coatedon their interior. walls with` .desired fluorescent salts or phosphors. Limitations in .this-respect are imposed almost entirely` by the ingenuity ofv theoperator..

It may-.be noted atthis point that probably;

emissivecoating,. can.. carry only about. 0.04'

ampere ot. electr-icrcurrent per squareinch 4of surface area.- Without.l detrimental overheating;

Through.- ther simple expedient of coating the working-surfaceof this electroder with electronemitting--coating such-- as -strontiu-rn or caesium oxide' or' the Alike,l .electron-emission' can be i11- per square inch.

While- 'tl-1ese,current densities aresuiicient, in

large measure, for-normal operation of the tube in4 actual practice-once the'tubehas been properlymanufactured-g theffdualI drawback is -still found to existithat; 1.. The tubeelectrodes must be too4 large andY too-long Whenhsubstantial currents arercarried, i.- ex; large-sizedtubes, and 2, the electrodesare detrimentally `over-heated and will give' rise' to -asputtering` deposit on the vitreous Walls-of theftubewcontainer during preliminary manufacture unlessfiexternal heating `equipment i' is *employedt Anw important "objectif omy'. invention, therefore; isf-torprovide-va.v methoduof. prelirninarilyy processing uorescent; and like space discharge tubes so as to avoid in substantial measure the many disadvantages and deficiencies of the prior art, and at the same time, to provide a technique whereby substantially al1 residual moisture and occluded gases are removed from the tube by the internal ionic bombardment method in the substantial absence of costly and space-confining ing external auxiliaries, and in which the electrodes alone participate in the production of the required elevated temperatures, without, however, any appreciable increase of the electrode size over that heretofore prevailing, without appreciable damage to the electrode itself, and without sputtering of the electrode material on the wall of the tube, the electrode being fully capable of satisfactory and efiicient operation during the subsequent normal use of the tube.

As a rst step toward solving the many difficulties hereinbefore recited, I decided upon the expedient of placing an oxide coating on both the internal and external walls of the electrode rather than upon the interior surface alone, as has heretofore been the case. I anticipated a theoretical doubling of the effective current carrying capacity, with but little increase in temperature and with no increase in the physical dimensions of the electrode. In point of fact, superior results were actualy achieved in practice. The current-carrying capacity was by no means doubled, but it was appreciably elevated. However, while satisfactory removal of moisture and occluded gases could be achieved, the ternperature of the electrode was raised to such values that highly skilled operators were required in carrying out the manufacturing operation. Moreover, some sputtering in the region of the mouth of the electrode was likewise observed. It was apparent that more was required in order to produce entirely satisfactory results.

I thereupon conceived the expedient of providing an electric iield in the region of each electrode, auxiliary to that. maintaining between the electrodes themselves when the later were subjected to alternating potentials, and asserting its presence between the electrodes and the adjacent walls of the tube container. Such field, properly designed, will effectively prevent the deposition of metal on the tube walls and at the same time will channel the electrons from the adjacent electrode, during the negative half cycle of that electrode, toward the opposed electrode, thus resulting in the greatest possible ionization at maximum energy of the electrons.

I provided this iield, in a first embodiment of the final form of my invention, by encircling the ordinary shell electrode by several turns of wire, which spiral anode was concentric with and spaced from the electrode by an amount suiiicient for the field to assert itself. The number of turns of the anode wire was such, and the spacing therebetween, that is, the pitch distance, so selected, that the electrons could flow freely therebetween. When this spiral element was constructed so that its turns projected slightly beyond the open end of the adjacent electrode, in the direction of the opposed spaced electrode. I observed that it fulfilled admirably its function as an anode. By coating this anode with an electron-emissive salt, I found that it participated in the electron-emission function of the main elec-- trode. By the use of such anode, I found that for the first time, the substantially increased current of the preliminary processing could be effectively carried by the electrode without damage to the latter and without detrimental sputtering and d deposition of sputtered metal on the walls of the tube.

And now having reference more particularly to that embodiment of my invention shown in Figures 2 through 4 of the drawings, the vitreous envelope l0, in the forni of an elongated. cylinder, has a re-entrant portion at each end thereof, terminating in an internal stem or press il. Support wire I3 is mounted therein, fast to lead-in wire Il', and carries at its free end cuph shaped or shell electrode it, of conventional size and dimension. In the preferred instance, to illustre-te. its length of the general order as its diameter, that is, it has no really major axis, other than that provided by the like contour. I provide an anode concentric with and disposed about the electrode iti throughout its length comprised of several t of wire i5. At least one turn of this wire projects beyond the outer edge of the electrode in the direction of the opposed electrode. In the preferred embodiment, the free end of this spiral l5 is connected to e1 trode Hl at point H5. The other end of the wire i5 is connected at E8 to the lead-in wire U5. 'Iubulature l2 serves for the evacuation of the tube I0.

To illustrate, it is desired, in order to compete eectively with the known hot cathode .fluorescent tubes, to operate with a cup-shaped electrode having an internal developed area of approximately l.2 square inches and an effective inner diameter of about of an inch. Such an electrode, capable of operating 25 mm. diameter tube, must function satisfactorily at a current density of 0.36 ampere.

Ordinarily, it must be effective carrying tem- `porarily, and without detrimental overheating and without detrimental sputtering, a current ranging from about 2.0 to about 2.5 amperes during the preliminary processing of the tube.

Coating both the inside and the outside surface of the electrode with electron-emissive oxides, I found a partial solution of the problem confronting me was obtained in that proper current intensity could be achieved during normal operation at an electrode temperature ranging from about 500 C. to 600 C. However, as has hereinbefore been stated more generally, such electrodes could not be successfully employed in preliminary processing of the tube under bombardment, and some detrimental sputtering was observed to take place.

I thereupon constructed the spiral l5 as hereinbefore referred to. In the instance undergoing description, I imparted a diameter of about of an inch thereto, in that instance where a shell is employed at 3/8 of an inch diameter, leaving a free space of about its of an inch around the shell. Upon coating the wire as well as the shell and thereafter processing several tubes by the glass-heating-bybombardment process, uniform results were achieved. I was able to raise the current in the tube to 2,5 amperes without undue risk. Upon shorting leads I6 and l'! (Figure 2) so that the shell ld and wire l5 operate in parallel, I found that the entire shell assembly could be raised to the required temperature of 750 to 800 C., solely by way of internal bombardment without damage to the electrode construction.

I constructed the wire spiral l 5 of pure iron, although nickel, molybdenum, er any other heatresisting metal could be effectively employed. In the present instance, the wire has a diameter of 0.031 inch and the pitch distance is about 1/8 of an inch. My observations have disclosed that theserdmensionsfare not; critical,..and that the design of the electrode is not limited 'in themannerrasfisftrue of theiconstruction of lamentary electrodes... Thus,. a shellf of 1.2squareinch internal-area I ndf-will work just aseeiectively with 0.3,.0.35 or"-.4 ampere current.

It 'has-'fbeen explained uhereinbeiore f how-`A a tube electrode :prepared in lthefmanner ijust described will operate :with ya high degree yof .effectivenessinthe preliminary :manufacture offthe tube. It will lnowfbe .describedhow such an electrode 013-- erateseffectively during the .L normal use of the tubeA after ithasbeenf.V sold and placed into'operation l-Iereragain;y justas in the 'instance 'heretofore described, effective contrasts will. be drawn betweenffthe prior'art cold cathode tubes on the one hand and my new4 electrode construction on the other hand.V

Assuming therefore th^t a shell electrode, properly oxide coated; andin a tube which has been properly heat treated, and-assuming an inside emitting surface of said electrode' offy say 1.2 square inchespthen assuming the shells to carry their loadaccordinghto Claudes law to the maximum rate` of 0.043 amperev per square inch, atotal permissible amperage of\0.0516"ampere can be achieved.1 Long-tube lifewill be achieved; and with af space Aoffabout-ly linches between the opposed electrodes; therefwillbe about 320evolts drop across theelectrodes when Aoperating atalternate currentafter initiallystriking` at a voltageA in the region of about 550 to 600 volts. The energyoutput of such as tubeisvery low,l ranging froml l10 to l2 watts with a tube of 'l0-25 mm. diameter.l That is; brilliancy is quite low. To obtain avhigh degree of illumination over a wide area -a number of lamps'must be strung ltogether inseries.

Increase in wattage consumptionof the tubes is indicatedas by passing greater current therethrough, thereby boosting the surface' brilliancy, decreasing` the operating voltage; and cutting down the 'operatingcosts Thus, to illustrate, upon increase of the'current from the 0.0516 vvalue herein-before referred to, to say about 0.150 ampere; the lamp brilliancy will be appreciably greater,- the operating voltage will drop tc about 2530""volt's` orthereabouts, and fewer lamps will beneeded'to maintain the same level of illuminay tion.

When'I surrounded this electrodewith al few turns *of pure iron wire,` all as'described heretofore in connection with spiral I5', and spaced the same fromr theshell I4's0 as to `provide enough space for the electronsv to move rather freely around it, I achieved the results'which I sought.V

My observations were that the spiral I5 served as the anodeof a sort of filamentary electrode when the last turnthereof was vextended slightly beyond the end of "the shell I4.v By that,l I mean no sputtering occurs on the inside glass wall around' the electrode. Moreover, I observed that no .cathode glow occurred on the outside Wall, this advantageous phenomenon being ,due in all probability to the electric iield set up-by theanode in phase with and of the sameinstantaneous` pon larity and potential as that of the shell I4. Thirdly, I observed that the anode emits electrons by eld emission in appreciable quantity. In fact, at theA initial-stage-ofbombardment the anode. is made to supply the current thatthe cathode can-not supply. Thus, with ionic bombardment takin-gfplace.1inside the shell, Ibelieve it to bea .fair` assumption 1 that the: 0.36 rampere fromsfthe `interior of the shell I4 and about 0.06-

amperef from; theexteriorof VtheA shell and from thei anode. Final-ly, I observed no cathodeglowl to .occur on' -orat thewire ,anodey -I 5.

In this embodiment, and under these conditions of operating,=;,I .iind vthe operating temperature oti'the fshell suchthat this latter assumes a dull, redicolor; indicating :a temperature of -about 600 C'.. While it'is4 true'fthat I operate my cathode shell at afsubstantially higher temperature than has heretofore been suggested according to,

Claudes. law; this value nevertheless falls far shortof thechotcathode operating temperature adverseaturns of the wire, I havingl observed that:

inzpracticefthis distance is suilcient to prevent sputtering ofthe-electrode.material on the glass walls of the tube;`

I have furthereobserved that during thenormal operation of the tube, the interior of the vitreous Wall'l becomes constantly charged to a negative potential, as indicated schematically in Figure 5r Now,`she1l I5 is alternately positive and negativel during.' each .half-cycle of the energizing current; whennpositive, .the'velecticfield createdthereby attracts the electrons emitted bythe electrode; Duringthehegative half-cycle of the charging current, however,` all the parts coming under theinuenceof the electricfleld of spiral I5 aref'attracted thereby, asare many ions createdA by# therJ electron bombardment and thereupon coming'nto th'e'inuence' of the eld of electrode I4. It'is the extreme mass ofthese ions which:intlargemeasure accounts for the sputtering which has heretofore-been so detrimental.

Referring'gnow to Figures 3 and 4, annular spacesnafz-bfandl cd are established between shell, III/an'dwire'i, and between wire I5 and tube I0,respectively;` When elements I4 and-I5 are positive, then the region a-b carries a positive charge'Asa result thereof, it not only attracts electrons thereto, but at the same time either repulses the positive-'ionsfor at least serves as a brake therefor. This lowered momentum resultsY inltheir being less destructive when they impact onthe shell I4. Space c--d is likewise f chargedfrpnsitively;atthis time, and thus to a tory, positive vions do not reach this electrode,

Whilethose which dobombard thereagainst have much `less disruptive-effect, due to their lowered velocitywand hence diminished momentum.

Now'whenthe-'elements I4 and I5 are negatively charged, a profusion of electrons escape from lthe=interior"of.shell I4, while lesser quantitles escape' from.' both thev exterior surface of shell"l4-and from wire anode I5. It should be notedthat elements I4 andi5 are simultaneously at' `tl'le-.esarneenegative-fcharger Accordingly', field a-b becomes negatively charged so that a strong electric field of negative potential is formed around electrode I4. Those electrons which escape from region a-b progress to a large extent towards the opposed electrode, which normally is positive. These electrons carrying over from the field a--b form a negative shield around the column of electrons escaping from the open end of electrode I4, this screen contracting the pencil of electrons and shielding the outer edge of this electrode, so that no sputtering can occur at that point. Simultaneously, the region c-d becomes negatively charged so that most of the migratory positive ions are attracted and as well, a strong tendency exists to neutralize the charge of the glass surrounding the electrodes.

I have just described the manner in which the presence of the anode participates in the establishment of electric fields which during the positive half-cycle of current flow shields both the electrode and the adjoining region of the glass envelope from ionic bombardment, and which during the negative half-cycle of current flow effectively dissipates the negative charge from the tube walls and directs the pencil of electrons towards the opposed, positive electrode.

It will now be in order to discuss briefly the advantageous results attendant upon the operation of the shell electrode at comparatively high temperatures during the normal service of the tube. By operating the electrodes I4 and I5 at elevated temperatures which while substantially higher than those heretofore customary, being in the neighborhood of 500 to 600 C., it is insured that the metal will not be softened to a dangerous extent. Nevertheless, these temperatures are suciently high to reduce the work barrier or function of the emitting surface of the electrode so that a much larger quantity of electrons can be emitted from the electrode surface than has heretofore been possible under cold cathode operation. I accomplish this simply by bombarding the electrode by a combination of electrons and positive ions until electrode temperature is reached where the work barrier is substantially lowered and an electron emission is achieved with substantially less cathode drop than has heretofore been possible.

While the form of tube shown in Figures 2 through 4 proves initially satisfactory, I have found that some difliculty was encountered from time to time in connecting the shell I4 and the spiral I5 together at I6. To illustrate, upon undertaking to install my new electrode construction into a tube of 38 mm. diameter, then the space c-d (Figures 3 and 4) becomes much larger than in the case of tubes of 25 mm. diameter. The desirable field action is diminished greatly in such instance, due probably to the negative charge on tube I being too remote from spiral I 5. In such instance, electron emission appears not only around spiral I5, but also around the interior and exterior of the shell I4. Inasmuch as the weld I6 serves only to support spiral I and to keep it in properly spaced relation to electrode III, I find it convenient in certain instances to replace the wire shield I5 with a cylindrical anode I8 closely encircling shell electrode I4, being spaced therefrom to a slight extent as illustrated in Fig. 1.

I prefer to construct this ring anode I8 of thin sheet metal having a thickness of say 0.005 inch. It may, however, conveniently be made of closely wound wire or with closely woven metal sheeting. The plain sheet metal I find, however, to be quite cheap to manufacture and to give entirely satisfactory results.

A support wire I9 carries ring I8 while support wire I3 carries electrode I4. These support wires I3 and I8 extend through press II and are connected with external lead-in wires I6, I'I.

In experimenting with this new construction, I found that it was entirely unnecessary in order to obtain the desired avoidance of sputtering deposit on the glass tube I0 for the anode to extend the entire length of the cathode I4. Moreover, I established that the distance which I call E, between the open end I4' of shell I4 and the in- Ward'edge I8 of cylinder I8 was critical at least to a certain extent. I found that this distance should vary between y'0.2 to about 0.4 times the diameter of the ring I8 itself.

When so constructed, all the desirable results heretofore sought I found to be admirably achieved. To satisfy myself that such was the case, I took occasion to measure the current established between lead-in I6, I'I when lead-in I'I was connected to the source of power. I found that when a total of 350 milli-amperes current flowed through the electrode I4, then about 75 to milli-amperes current was flowing through these two lead-ins. Inasmuch as this current was measurable with a direct current meter, this established that a uni-directional alternating current ow had been achieved through rectification in the mercury arc. Moreover, since at al1 times the lead-in I'I was positive with respect to lead-in I6, it is reasonable to assume that electrons ow from anode I8 through lead-in I6 to lead-in II. Upon disconnecting lead-in I6 from lead-in I'I, and thereupon measuring the potential difference therebetween, this was found to attain a value of from 25 to 60 volts, direct current. It appears to be a reasonable conclusion that this potential exists between the gas column and the anode I8. It may thus logically be concluded that the higher the operating current, the lower the voltage across the discharge gap, a phenomenon which is found to take place in actual practice.

Upon contrasting the operation of the construction according to Figure 1 with that according to Figures 2 through 4, inclusive, I concluded that while in the embodiment according to Figures 2 through 4, improved results are achieved if the spiral I5 is oxide coated, it is unnecessary in the embodiment, according to Figure 1, to coat the ring anode I8 with an oxide coating. Only the interior surface of the shell I4 need be coated for electron emission. I nd that using the open stop-cook technique of manufacture, then with a ring anode having a width of 1A an inch and a diameter of 126 of an inch, and employing a shell of 1,259 inches developed surface arc, I can by internal ionic bombardment, properly heat a tube of 8foot length and of 25 mm. diameter in very easy and rapid manner, with sensible reduction in manufacturing costs and with the production of a clean and long-lasting lamp.

The practice of my invention makes it readily possible, for the first time, to employ a shell type cathode of minimum dimensions with increased current carrying capacity under cold cathode opoperation. My new technique makes it entirely possible for the electrode to have an initial current-carrying capacity sufficient to heat both the surrounding glass envelope itself and the electrode by internal bombardment to a temperature sufficientlyhigh to drive off retained moisture and occluded gases, while avoiding abnormal sputtering of the cathode material. I nnd that the heat imparted to the shell surface by ionic bombardment increases the kinetic energy of the electrons in the cathode metal, thus freeing more electrons through a weakened potential energy barrier. Moreover, the cathode drop across the electrodes is appreciably diminished, while both the striking voltage and the operating Voltage of the arc itself are substantially lowered. All these, along with many other highly practical advantages, attend upon the practice of my invention.

It may be noted that the phenomenon is observed that the working area, that is, the electron-emissive area, of my electrode is large during the initial bombardment process, but is thereupon decreased during the normal operation of the electrode. This phenomenon is attendant upon the practice of my invention according to Figure 1, simply by coating only the interior surface of the shell I4. With such construction, then at the very outset of the preliminary bombardment the combination of the three factors comprising the excess of bombarding current, the high work function of the surface of the shell, and` the elevated air pressure or gas pressure maintaining in the tube serve to spread the bombarding arc over the entire electrode assembly. This latter is brought to a dark cherry red temperature approximating '700 C. Thereupon, as the original coating of salts of electron-emissive substances is reduced to highly emissive oxides, the shell I4 progressively takes the current load until it attains a light cherry red temperature corresponding to about 850 C., and thereupon carries substantially the entire tube current, amounting to about 2.50 amperes, When this current levels off at a steady value, then the tube is ready to be cooled, filled and sealed off.

While, in general, the character of the salts employed are sufciently oxidizable to assume excellent elimination of moisture and occluded gases I nevertheless find it advantageous in some instances to subject the electrodes, with their electrin-emissive salt coatings, to a reducing treatment prior to assembly in the lamp. In this way I assure complete elimination of moisture and occluded gases in the subsequent operations wherein the electrodes are brought to a light cherry red heat.

Since many embodiments of my invention will readily occur to those skilled in the art, once the broad aspects thereof are disclosed, it will be understood that all matter described herein,

10 or shown in the accompanying drawings, is to be as illustrative and not in a limiting sense.

I claim as my invention:

1. As a new article of manufacture, a space discharge tube, comprising, in combination, a glass envelope, a shell electrode sealed therein in the form of a cylindrical cup with an open face and a closed bottom and having an electronemitting oxide coating on the interior thereof, and a cooperating cylindrical ring anode also sealed therein and encircling said electrode in uniformly spaced relation thereto and extending therebeyond a slight distance.

2. As a new article of manufacture, a space discharge tube, comprising, in combination, a glass envelope, a shell electrode sealed therein in the form of a hollow cylindrical cup with open face and closed bottom, and a cooperating cylindrical ring anode also sealed therein and encircling said electrode in substantially uniformly spaced relation and extending therebeyond a slight distance, the space from the outer edge of the electrode to the inner edge of the ring anode ybeing between approximately 0.2 and approximately 0.4 times the diameter of the anode.

3. As a new article of manufacture, a space discharge tube, comprising, in combination, an elongated glass envelope, a pair of cup-shaped electrodes with cylindrical walls and closed bottoms sealed therein at opposite ends thereof with the cupped openings facing each other, each electrode having an electron-emitting oxide coating on the interior thereof, and a corresponding pair of cooperating cylindrical ring anodes respectively uniformly encircling said electrodes in the regions of their open ends only.

RALPH CORTESE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,946,336 Smith Feb. 6, 1934 1,971,907 Found Aug. 28, 1934 2,008,066 Ende July 16, 1934 2,020,393 Woolrich Nov. 12, 1935 2,085,561 Wiegand June 29, 1937 2,233,741 Kirsten Mar. 4, 1941 2,259,947 Blackburn Oct. 21, 1941 2,314,134 Eknayan Mar. 16, 1943 2,375,808 Miller May 15, 1945 

